Over the decades, wireless communication systems have become more and more technologically advanced, with performance increasing in terms of smaller size and robustness, among other factors. The trend toward better communication systems puts ever-greater demands on the manufacturers of these systems. These demands have driven many developments in microwave technology.
Looking at some of the major developments historically, the early 1950's saw development of planar transmission media, creating a great impact on microwave circuits and component packaging technology. Developments in the engineering of microwave printed circuits and the supporting analytical theories applied to the design of striplines and microstrips contributed to improvements in microwave circuit technology. A historical perspective on some of the developments of microwave integrated circuits and their applications is provided by Howe, Jr., H., "Microwave Integrated Circuits--An Historical Perspective", IEEE Trans. MIT-S, Vol. MTT-32, September 1984, pp. 991-996.
The early years of microwave integrated circuit design were devoted mostly to the design of passive circuits, such as directional couplers, power dividers, filters, and antenna feed networks. Despite continuing refinements in the dielectric materials used in the fabrication of such circuits and improvements in the microwave circuit fabrication process, microwave integrated circuit technology was characterized by bulky metal housings and coaxial connectors. The later development of case-less and connector-less couplers helped reduce the size and weight of microwave integrated circuits. These couplers, sometimes referred to as filmbrids, are laminated stripline assemblies that are usually bonded together by fusion or by thermoplastic or thermoset films.
Traditionally, the size of a coupler in the X-Y-plane is governed by the length of the stripline sections being coupled. A coupler designed to perform over wide bandwidths requires additional sections of coupled striplines, which would further increase the overall size of the coupler. Furthermore, since the length of the coupled sections is inversely proportional to the operational frequency of the coupler, a coupler designed to operate at lower frequencies would have longer stripline sections. Coupled lines are often meandered to decrease their effective outline size.
Today, the demands of satellite, military, and other cutting-edge digital communication systems are being met with microwave technology. The growth in popularity of these systems has driven the need for compact, lightweight, and surface-mountable packaging of microwave integrated circuits. Although advances in microwave integrated circuit technology, such as those outlined above, have helped decrease the size, weight and cost of the circuits, it would be advantageous to decrease the size, weight and cost of such circuits even further. In sum, present technologies have limitations that the present invention seeks to overcome.